As a general concept, industrial heat treat processes (such as annealing, carburizing, clean hardening, etc), and various heat driven process (such as sintering, brazing, purging, blanketing) require a furnace atmosphere or furnace gas having certain characteristics, or generally speaking, a certain potential for producing carbon. The gas composition is well defined, as for example by Class designation specified by the American Society for Metals. Thus, the carrier gas used in carburizing is an endothermic base carrier gas specified by ASM as a class 300 gas typically having a gas composition of approximately 40% N.sub.2, 40% H.sub.2 and 20% CO. A typical gas composition would be:
______________________________________ Gas Element Percentage ______________________________________ H.sub.2 40.4% CO 19.8% N.sub.2 39.0% CH.sub.4 0.5% H.sub.2 O 0.2% CO.sub.2 0.1% ______________________________________
Conceptually, there are three ways to supply at least some of the furnace atmospheres. First the furnace atmosphere can simply be supplied as a gas and stored on site for use in the process, i.e., liquid nitrogen. However, certain heat treat processes require tighter and tighter control of gas compositions so that even in the inert applications, such as nitrogen, oxygen presence of low PPM levels cannot be tolerated. It is possible, that even with commercial grade nitrogen stock, oxygen concentrations of 20 PPM can occur which exceeds the low oxygen concentrations now being specified.
A second way to form the furnace gas, is to react or dissociate a liquid, usually in the presence of heat, to produce its gaseous components. A typical example is ammonia which is dissociated to produce nitrogen and hydrogen. This is a commercially acceptable process but is somewhat costly, requires liquid gas storage facilities and does have a tendency to produce trace elements of the liquid as a gas in its original form, i.e., NH.sub.3.
The last commercially-acceptable method for producing furnace atmospheres, and the one to which this invention relates, is the use of gas generators to develop a variety of furnace gas compositions by reacting a hydrocarbon fuel, such as natural gas (methane) with air.
Traditionally gas generators are defined as either exothermic or endothermic. An exothermic gas generator means a generator where the air is mixed with natural gas at ratios of anywhere between 6 to 9 parts air to 1 part natural gas (stoichiometric combustion occurring at about or slightly less than 9). At these ratios the air and gas burn with the products of combustion giving off heat and thus the designation of such generators as exothermic. Exothermic generators typically produce lean or inert atmospheres without (or with very little) combustibles and their applications typically involve annealing, furnace purging, brazing etc. Endothermic generators typically involve generators where the air to gas ratio is typically less than 6 to 1 and more specifically is generally set at 2.5 to 1. When air and CH.sub.4 are supplied in such ratios, heat must be inputted to the mixture to dissociate the gases to their elemental form. As explained in the prior art patents, the gas reactions resulting during the dissociation of the gases are not entirely endothermic (just as certain gas reactions in the exothermic generator are not entirely exothermic). However, the term, endothermic, is used to describe such generators since heat is initially supplied to the air/natural gas mixture to raise the temperature of the mixture prior to entering the catalyst bed within the generator. Endothermic generators are typically used to produce Class 300 gas and any gas which is reducing or has combustibles such as H.sub.2 or CO. Within the heat treating art generators which lack combustibles are viewed as non-reducing. Generated gases which contain less than 4% combustibles or 1% oxygen are viewed as inert.
Because a number of heat treat processes require certain percentages of gases it is not unusual to couple exothermic generators such as Surface Combustion's DX generators with endothermic generators such as Surface Combustion's RX generators. Nor is it unusual for specific process applications to use molecular sieves or desiccants to remove or reduce certain elements, such as CO.sub.2 from the gas, in installations where HNX generators are employed such as in tin lines and other coating lines.
In general summary of the natural gas generator art as it now exists, two fundamentally different kinds of generators must be used depending on whether the gas composition is to be non-reducing or reducing (rich in combustibles). Further, depending on process requirements, a wide variety of additional equipment items are used to produce a wide range of gas compositions resulting from the product gas compositions produced by the endothermic and exothermic gas generators blending "RX" and "DX" gases. Thus, the existing generator art lacks flexibility to produce a wide variety of gas compositions. This results then in the user purchasing a relatively large number of expensive equipment items or adopting alternative gas producing approaches which have their own problems. Further, process control requirements are becoming increasingly tighter for gas compositions. As already noted, the presence of oxygen in certain inert type annealing atmospheres which can adversely affect the metal as an oxidizer, now require oxygen content to have no more than 1 PPM.
Apart from any considerations relating to gas generators, membrane nitrogen generators remove nitrogen from air by passing the air through a membrane which separates certain elements from the air. Specifically, a gas stream comprised substantially of nitrogen with varying amounts of oxygen can be produced. At this time, none of the known membrane systems can economically produce from air a substantially pure stream of nitrogen without the presence of oxygen in some amount. Thus these types of nitrogen generators are not sufficient, in and of themselves, to produce a stream of totally pure nitrogen which can be used as in inert furnace atmosphere for a relatively wide variety of purposes within the heat treat art.